Remote laser beam welding uses mirrors in a processing head to manipulate and guide a laser beam to a workpiece. Such a process enables a highly productive and flexible production line layout, and hence makes laser welding production more cost effective. More particularly, in the welding of auto-body components, remote laser welding greatly simplifies the process procedures. Lap and butt joints are common joining configurations in such components. Advantageously, laser beam welding technology has a low total heat input and hence causes low distortion to the components. Therefore, remote laser beam welding provides a welding technology for producing lighter and more compact auto-body structures.
The increased use of zinc or zinc alloy coated metal sheets in auto-body components for enhanced corrosion resistance creates a demand for an acceptable welding method to join these kinds of sheets. Laser beam welding has an advantage of low total heat input and thus causes a low distortion to the zinc-rich coating on the sheet. Therefore, laser beam welding is being evaluated as a desirable joining technique for such sheet steel in butt and lap joint configurations. However, a problem arises with welding these materials in the lap-joint configuration due to the low boiling point of zinc (906° C.) compared with the melting temperature of steel (˜1550° C.).
If there is no joint clearance between the sheets the zinc vapor during welding can only escape through the molten welding pool, and this typically results in excessive weld porosity or complete expulsion of the weld metal. In order to make a good quality weld, there are, in principle, two solutions to get around this problem: (1) creating a zinc-vapor venting channel during welding; or (2) removing the zinc coating in the welding pass. Both of these approaches need additional techniques to be realized. Many techniques have been developed to provide a gap between sheets to perform laser welding and to remove the zinc coating at the welding spot. These approaches typically require the use of supplementary components or spacers and are difficult for maintenance in the production line, since additional equipment is required to create a gap or remove the coating. This will incur a significant expense and increase production time.
Providing a small gap between the sheets has been proven to be the most practical technique to create a zinc-vapour venting channel. However, to maintain a small gap between the sheets, especially in a production line, can be challenging.
PCT Publication WO 99/08829 discloses a method for conventional laser beam welding of a zinc-coated steel sheet in which protuberances are generated on one of the joining sheets by a laser dimpling technique. Laser dimpling has been proven to be a viable technique to generate dimples for providing a vapour venting gap at the welding location. This is a flexible pre-process which is not limited by the welding location and the shape of the components. In spite of these benefits, this preprocess still adds production cost to the welding process. In order to minimize the additional cost, the laser dimpling process needs to be carried out as fast as possible. Therefore, this dimpling technique can not be readily applied to remote laser beam welding processes due to its limited process speed.
It is desirable to provide an efficient solution or technique to generate a gap in remote laser beam welding.